Electrical excitation of gas discharge lasers, such as CO.sub.2 lasers, requires the use of an excitation source with the following capabilities:
1. Provision of a high voltage to create electrical breakdown in the column of gas that forms the active medium for the laser;
2. Provision of sufficient impedance in series with the gas discharge to maintain controllable current flow through the laser despite a negative dynamic impedance characteristic of the glow discharge in the laser; and
3. Provision of means for repetitively cycling the discharge between the OFF state, i.e. no current flow, through the breakdown phase, to the ON state with regulated current flow, and back to the OFF state again.
In the past, CO.sub.2 laser excitation has been accomplished by the use of a high voltage source together with a series current regulator. The current regulator typically consists of a high voltage vacuum tube triode or tetrode connected in the current path. As is well known in the art, a higher peak power can be obtained from a gas discharge laser if the voltage applied to initiate electrical breakdown is substantially higher than that required to maintain a continuous discharge of the laser after the breakdown occurs. For example, a "no-load" voltage two to three times higher than the continuous discharge voltage can be used. However, if the continuous voltage source maintains the "no-load" voltage level when the laser is run continuously, other components of the excitation circuitry, such as the current regulator, must dissipate excessive power. While this approach is feasible, it is clearly not economically viable.
Reference is made to U.S. Pat. No. 4,061,986 assigned to the assignee of the present application, in which an improvement to conventional excitation schemes is disclosed. This improvement includes adding a second high voltage supply having high internal impedance to the primary power supply. When the electrical breakdown in the laser occurs and current starts to flow, the voltage applied by the second supply falls to a low value. There after the primary power supply provides the discharge voltage to the laser. This allows more rapid breakdown and higher initial current than otherwise obtainable, and permits "enhanced-pulse" or "super-pulse" operation of the laser. U.S. Pat. No. 4,061,986 provides additional background on the enhanced pulse mode operation of gas discharge lasers.
The improved power supply of the above referenced patent is capable of providing both continuous operation and pulsed operation over a wide range of current, pulsewidth, and repetition frequency. As such, that invention has found application in a wide variety of uses. Many CO.sub.2 lasers, however, are used in dedicated and narrowly defined applications such as ceramic scribing, or printed circuit board drilling, where the laser operates only in the enhanced pulse mode and the pulsewidths are predetermined and fixed, or at most varied only within a small range. Furthermore, there are certain applications, such as CO.sub.2 laser surgery, where the laser must be capable of continuous operation, as well as enhanced pulsed operation. However, in the pulsed operation mode, the pulsewidth is predetermined and fixed.
In the above more narrow applications, use of a laser having the cost, size and complexity of a laser having the improved power supply as disclosed in the above-referenced patent becomes difficult to justify. There is therefore need for a power supply which is capable of continuous operation as well as enhanced pulse operation, where the pulsewidth in the pulse mode is predetermined and fixed, and which is significantly less costly, smaller and less complex than previous power supplies.